Power over Ethernet (PoE), in accordance with both IEEE 802.3af-2003 and IEEE 802.3at-2009, each published by the Institute of Electrical and Electronics Engineers, Inc., New York, the entire contents of each of which is incorporated herein by reference, defines delivery of power over a set of 2 twisted wire pairs without disturbing data communication. The aforementioned standards particularly provide for a power sourcing equipment (PSE) and a powered device (PD). There are two methods of injecting power over Ethernet: Endspan; and Midspan. In Endspan injection, the PSE is provided at an Ethernet hub, or switch. The PSE provides power over the data wires connecting the Ethernet switch and the PD. In Midspan injection, the power is provided through a midspan injector arranged to inject the PSE supplied power into the data wires between the Ethernet hub or switch and the PD.
FIG. 1 illustrates a high level schematic diagram of a prior art PoE midspan injector 10, comprising: an Ethernet device side data port 20 comprising a plurality of pins 25; a powered device side data port 30 comprising a plurality of pins 35; a pair of auto-transformers 50, each comprising a coil 55 wound around a magnetic core 60; a plurality of DC blocking capacitors, denoted CB; a plurality of pairs of data wires 70, denoted 70A, 70B, 70C and 70D, respectively; a power reception port 80; and a power return port 90. In one non-limiting embodiment, magnetic core 60 of each auto-transformer 50 comprises ferrite.
PoE midspan injector 10 is in communication with an Ethernet device 100, such as an Ethernet hub or Ethernet switch, via Ethernet device side data port 20. PoE midspan injector 10 is further in communication with a PD 110 via powered device side data port 30. Each pair of pins 35 is coupled to a respective pair of pins 25 via a respective one of data wire pairs 70A, 70B, 70C and 70D. Power reception port 80 is coupled to a power output of a power sourcing equipment (PSE) 40 and power return port 90 is coupled to a return of PSE 40. PSE 40 is illustrated as being external of PoE midspan injector 10, however this is not meant to be limiting in any way and in another embodiment (not shown) PSE 40 is provided within PoE midspan injector 10.
Power reception port 80 is coupled to a center tap of a first auto-transformer 50 and the first and second ends of first auto-transformer 50 are each connected to a power node NP of a respective wire of data wire pair 70C, with power node NP for each wire of the respective wire pair defined as an electrical connection point between pin 25 and pin 35. Power return port 90 is coupled to a center tap of a second auto-transformer 50 and the first and second ends of second auto-transformer 50 are each connected to a power node NP of a respective wire of data wire pair 70D.
Each of data wires 70C and 70D is arranged to couple the respective pin 25 to the respective pin 35 via a respective DC blocking capacitor CB, DC blocking capacitor CB coupled between power node NP and pin 25.
In operation, PSE 40 is arranged to provide direct current (DC) power to PD 110 via first and second auto-transformers 50 and the respective data wires 70C and 70D. In particular, PSE 40 provides power to power nodes NP of data wires 70C, the return of PSE 40 being through power nodes NP of data wires 70D. Blocking capacitors CB are arranged to block the output DC power of PSE 40 from reaching Ethernet device 100, while allowing data signals to pass therethrough. PoE midspan injector 10 is illustrated as providing power from a single PSE 40 over data wire pairs 70C and 70D, however this is not meant to be limiting in any way. In another embodiment (not shown), PoE midspan injector 10 is arranged to provide power from two PSE 40s over all four of data wire pairs 70A, 70B, 70C and 70D.
The impedance of an electric coil is a function of its inductance and parasitic capacitance between windings and between the windings to the core.
The impedance of the inductor only, i.e. without the parasitic capacitance thereof, is given as:XL=2*π*f*L  EQ. 1where f is the signal frequency and L is the inductance of the coil. The inductance of the coil, constructed for example as a Toroid, is given as:L=(μ0*μr*N2*A)/l  EQ. 2where μ0 is the magnetic permeability of air, μr is the relative magnetic permeability of the coil core material, N is the number of coil windings, A is the area of the cross-section of the coil and l is the length of the coil in its winded state. The impedance of the parasitic capacitance (inter winding capacitance and stray capacitance) is given by:XC=1/(2*π*f*C)  EQ. 3where C is the parasitic capacitance and f is the signal frequency. The capacitance is increased if the number of coil windings is increased. In general, the total impedance of the wound coil is XL in parallel to XC. Although the total impedance is more complex and may contain several combinations of inductance and capacitance, a simple construction of inductance and capacitance in parallel will be addressed for simplicity.
As shown in EQ. 1, the impedance of coil 55 decreases when the data signal frequency decreases. In order to prevent excess loading of data signals on data pairs 70C and 70D due to coil 55, the impedance thereof must be sufficient over the range of data frequencies. At low frequencies, a large inductance L is necessary to so as to avoid having a low impedance load across the respective data wires 70C and 70D. As shown in EQ. 2, increasing the inductance of coil 55 may be accomplished by increasing the number of windings and/or increasing the cross-section area thereof, however this increases the size of auto-transformer 50 which is undesirable. Magnetic core 60 is therefore provided to increase the magnetic permeability μ, and as a result increase the inductance, and therefore the impedance, of auto-transformer 50 without having to increase the number of windings and cross-section area thereof. Typically, coil 55 and magnetic core 60 are selected such that auto-transformer 50 exhibits an inductance of 160-600 μH, preferably greater than 500 μH, the inductance measured at a data frequency of about 100 kHz. The inductance of each of Ethernet device 100 and PD 110, which are in parallel to the inductance of auto-transformer 50, is typically about 160 μH, therefore an inductance of greater than 500 μH is preferred for auto-transformer 50 so that the inductance of auto-transformer 50 does not significantly impact the inductance of Ethernet device 100 thereby disrupting the data transmission over data wire pairs 70C and 70D.
10-gigabit (10G) Ethernet, also known as 10GBase-T, in accordance with IEEE 802.3-2012, published by the Institute of Electrical and Electronics Engineers, Inc., New York, the entire contents of which is incorporated herein by reference, defines Ethernet frame transmission at 10 gigabits per second. For 10G Ethernet, each data wire pair 70A, 70B, 70C and 70D should be able to transmit data at frequencies of up to 500 MHz, preferably up to 600 MHz, such that 2.5 gigabits of information are encoded and compressed into the 500 MHz signal. Unfortunately, at such frequencies the magnetic permeability μr of magnetic core 60 rolls off, thereby reducing the inductance of auto-transformer 50, as seen from EQ. 2. As a result, as seen from EQ. 1, the impedance of auto-transformer 50 decreases. Additionally, at high frequencies the parasitic capacitance between the windings of coil 55 further reduces the impedance of auto-transformer 50. Therefore, at 10G Ethernet data frequencies auto-transformers 50 will provide a low impedance load to the respective data lines thereby creating significant insertion and return losses in the transmitted data.
There is thus a long felt need for a POE Midspan injection method and apparatus which allows for 10G Ethernet data transmission.